1. Field of the Invention
The present invention relates to a stabilizer for preventing the thermal decomposition or reaction of an organic borate salt, a photosensitive composition comprising the stabilizer and an organic borate salt, a polymerizable composition comprising the photosensitive composition having added thereto a monomer having an ethylenically unsaturated bond, which has excellent heat stability and aging stability and is prevented from impairment of the polymerization reactivity, a photopolymerization initiator capable of exhibiting good polymerizability even in the presence of oxygen, and a polymerizable composition containing the photopolymerization initiator.
This application is based on Japanese Patent Applications Nos. Hei 9-229376 and Hei 10-199080, the contents of which are incorporated herein by reference.
2. Description of Related Art
A large number of studies have been made on organic borate salts as an important constituent element of a photopolymerizable composition. Examples thereof include an organic boron compound anion salt of an organic cationic dye (see, JP-A-62-143044 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-64-13141 and JP-A-2-3052) and a system of separately adding an organic cationic dye and an organic borate salt but not adding these in the form of a complex (see, JP-A-2-4804 and JP-A-5-194619). In any of these publications, a photo-polymerizable composition having high photosensitivity obtained by adding a monomer having an ethylenically unsaturated bond to the organic borate salt, is disclosed.
However, these photopolymerizable compositions containing an organic borate salt are not sufficiently high in the heat stability. For example, in a photopolymerizable composition containing an organic borate salt, the organic borate salt gradually decomposes at 0.degree. C. or higher and the decomposition is further accelerated at higher temperatures. As a result of decomposition of the organic borate salt, the photopolymerizable composition is reduced in the photopolymerizability and cannot be cured even with a sufficiently large amount of exposure.
Not only the decomposition of the organic borate salt in the photopolymerizable composition but also unintended polymerization reaction proceed in a dark place. Thus, the photopolymerizable composition is polymerized before exposure and cannot be used any more. Furthermore, as the acidity of the photopolymerizable composition increases, the organic borate salt is extremely reduced in the heat stability and seriously decomposes during the storage.
In order to overcome these problems, attempts have heretofore been made to achieve stabilization by controlling the structure of the organic borate salt, the structure or composition of the sensitizer or monomer having an ethylenically unsaturated bond, or the neutralization of the acidity of the composition. However, satisfactory effects have not yet been achieved.
With respect to the photopolymerizable composition containing a monomer having an ethylenically unsaturated bond and as the photopolymerization initiator system, a titanocene compound and a sensitizer, a photopolymerizable composition further containing a heterocyclic thiol compound so as to achieve stabilization of the titanocene compound is known (see, JP-A-9-5996). However, the effect of improving the stability of the titanocene compound is not yet satisfied.
On the other hand, radiation polymerization using a radiation-sensitive polymerization initiator is being used in various fields such as semiconductor relative, printing ink, printing plate, coating, adhesion and liquid crystal display. The radiation includes high energy radiation such as electron beam and X ray, and low energy radiation such as ultraviolet ray, visible ray and infrared ray, and the radiation is applied by means of a device (lamp) emitting respective rays according to the use purposes. The high energy radiation has a high polymerization initiating capability because of its high energy but is disadvantageous in the complicated apparatus or process necessary therefor, the high use energy and the adverse effects on the environment or an operator. However, due to tendency to use a photopolymerization initiator which generates radicals by light of a low energy radiation such as ultraviolet ray, visible ray or infrared ray, the dangerous high energy radiation is less required. In particular, the photoradical polymerization initiator can polymerize vinyl-base monomers with low energy to thereby promise high sensitivity and high-speed processing and accordingly, is being used very often.
However, it is known that if the radical polymerization is performed in the presence of oxygen, the radicals generated combine with the oxygen to convert into a relatively stable peroxy radicals not having the polymerization initiating capability, whereby the polymerization is inhibited and insufficient polymerization results. This phenomenon is particularly outstanding on the surface of the photopolymerizable composition in contact with air or in the case of a thin film.
To overcome this problem, a method of blocking oxygen by forming an oxygen-blocking film on the surface of the polymerizable composition so as to prevent the effects of oxygen, a method of performing the polymerization by placing the polymerization material as a whole in a nitrogen atmosphere, a method of adding a large amount of a photopolymerization initiator to maintain the sensitivity, a method of adding a tertiary amino compound and generating active radicals from stabilized peroxy radicals to allow the polymerization to proceed, and a method of using a photocationic polymerization initiator and not relying on radicals have heretofore proposed.
However, the above-described methods for overcoming the problem of polymerization inhibition due to oxygen are disadvantageous in that the costs of raw material, process and equipment are greatly increased, the sensitivity is low, the productivity decreases due to complicated processes, and stability or odor of the polymerizable composition or the product after polymerization is another problem.
Accordingly, the methods are limited in the use field or usage.